Jamie Foxx is being accused of slapping a woman with his penis nearly 16 years ago, and although she's now gone to police ... Foxx says it's an "absurd" lie, and he's going after her, legally ... TMZ has learned.

The alleged assault happened in Las Vegas back in 2002. According to law enforcement, the accuser told cops last week she and a friend were attending a party at Jamie's place when he tried to get her to perform oral sex. She claims when she refused, Jamie struck her in the face with his penis.

The accuser tells us ... after the alleged assault, one of Jamie's friends told her she had to get out of the house. She claims she went to a hospital the next day when she got back home to L.A. to get treatment for a severe panic attack.

Law enforcement sources tell us Vegas PD lists the case as an open investigation. However, the statute of limitations has long since run -- it's 3 years in Nevada.

Jamie's attorney, Allison Hart of Lavely & Singer, tells TMZ ... "Jamie emphatically denies that this incident ever occurred, and he will be filing a report with the Las Vegas Police Department against the woman for filing a false police report against him. The first time [Jamie] became aware of this woman's absurd claims about an incident that supposedly occurred 16 years ago was when TMZ contacted his representatives [Tuesday] about this story."

Hart adds, "The alleged incident was not reported to law enforcement in 2002, or at any other time in the last 16 years until last Friday because the incident never happened."

The accuser says the #MeToo movement inspired her to finally file the police report. She also claims she wanted the alleged incident recorded in case other accusers come forward.

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